hmp: Add name parameter to nbd_server_add

Extend the flexibility of the previous QMP patch to also work
in HMP.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20180109192802.17167-1-eblake@redhat.com>